Method for treating gamma ray detectors



United States Patent METHOD FOR TREATING GAMMA RAY DETECTORS Roy P. Mazzagatti, O. D. Stephens, and Archie S. Conners, Houston, Tex., assignors to The Texas Company, New York, N. Y., a corporation of Delaware No Drawing. Application December 29, 1950, Serial No. 205,958

6 Claims. (Cl. 316-20) This invention relates generally to the measurement of penetrative radiation and more particularly to a method for treating radiation detectors of the electrlcal pulse-producing type, such as gamma ray counters so as to reduce their background counting rate. The principal object of the invention is to minimize this bothersome and unwanted background without adversely affecting the sensitivity of the counter for the detection of penetrating gamma rays.

Although the invention is applicable for use in conventional gamma ray counters, it is particularly advantageous in connection with multi-cathode plate counters of the type described in the U. S. Letters Patent No. 2,397,- 071, granted March 19, 1946, to D. G. C. Hare. In that patent a counter is disclosed in which the cathode 1s formed of a plurality of thin, separated metal plates or disks arranged in parallel. The bank of plates is provided with one or more series of holes disposed in alignment and an anode wire is stretched along the longitudinal axis of each series of holes. As is the case with the conventional counter, the electrodes of th c Hare counter are disposed in a casing or housing containing an ionizable gas.

In the copending application of R. P. Mazzagatti, Serial No. 138,706, filed January 14, 1950, a method is described for reducing the background in a gamma ray counter in which the cathode is formed of tantalum. In accordance with that method, the cathode surface or surfaces are coated with tantalum oxide, preferably electrolytically, and it has been found that this coating prevents to a considerable degree spurious discharges caused by ionization of the gas caused either by cold emission of electrons from the cathode material or by the presence of radioactive impurities in the cathode metal.

The objects of the present invention are similar to those expressed in the above-mentioned Mazzagatti application, namely, the reducing of spurious counts or discharges so that the counting rate of the detector Wlll be substantially a true indication of the intensity of gamma rays which pass into the detector and strike the cathode material to eject electrons therefrom. In other words, it is the purpose of the invention to provide a method or procedure for the reduction of background or spurlous counting in gamma ray counters in those cases where the surfaces of the materials within the counters, particularly the cathode, are of such a nature as to give counts because of electron emission not due to penetrating radiation.

In accordance with the invention, the electrodes of the counter, i. e., the cathode and the anode, are subjected to a very hard vacuum, at an absolute pressure preferably less than one micron of mercury, while simultaneously a high alternating potentlal is impressed across the electrodes. This treatment is continued preferably for a period of from 1 /2 to 2 /2 hours, after which the vacuum and the alternating potential are removed and the electrodes surrounded with an ionizable, gaseous atmosphere.

For normal operation the cathode and anode electrodes are of course disposed in a suitable housing which may be of metal or glass and which contains a filling of a gas such as argon or a mixture of gases such as is described in the U. S. Letters Patent No. 2,512,773, granted June 27, 1950, to Gerhard Herzog and A. D. Garrison. In

the electrode treatment comprising the present invention,

the electrodes may be placed in some other container in which they are subjected to the vacuum and the alternating current potential, after which they are removed and mounted in the housing in which they are to be maintained during the normal use of the instrument. It is preferred, however, to mount the clean electrodes in their counter housing and then to produce the vacuum within that housing, while impressing the high alternating potential across them. In this manner, after the teatment has been continued for a desired length of time, and the vacuum and the high potential removed, the counter housing can be filled to the desired pressure with the ionizable gas and without danger of contaminating the electrode surfaces by touching them.

An example of this procedure is as follows: After the normal procedure for cleaning and fabricating a gamma ray counter, it may be found in testing that the counter has a high background counting rate due to surface conditions within the counter. The gaseous filling is removed from the counter housing and the interior thereof then subjected to a good hard vacuum of less than one micron of mercury pressure, while at the same time an alternating potential of 6000 to 8000 volts at a frequency of about 60 cycles per second is impressed upon the cathode and the anode of the counter. This condition is caused to be maintained for a suitable time, preferably about two hours, after which the counter housing can be refilled with the desired gas. The counter should then be substantially free of spurious or background counts. In one instance a multi-cathode plate counter was built in which the cathode metal was aluminum and during testing operations it was found that the background was so high that the instrument would be unsatisfactory, particularly when used for measuring gamma rays of low intensities. The counter was given the treatment described above, and it was found that the background had been reduced to a negligible amount.

Various reasons may be advanced for the eificacy of this method in improving the operation of a gamma ray detector. During the treatment, the alternating current potential across the anode and the cathode results in an alternating current electrostatic field between these electrodes, and this electrostatic field Will, of course, exert a force on any charged particle, or particles, existing on the anode or the cathode or between them. These particles may be loosely bound electrons on the surface of the cathode or the anode, and they will be forced, due to the electrostatic field, to disengage from their loosely bound positions, whereupon they will be collected by either the positively charged anode or the positively charged cathode, as the case may be at that instant. As stated above, this procedure is carried on for a suitable time until substantially all of the loosely bound electrons are collected. Then, when the detector is again placed in operation, no loosely bound electrons will exist to cause spurious counts or pulses during the operation of the detector.

Whereas a potential of 6000 to 8000 volts at a frequency of about 60 cycles per second for a period of time from 1 /2 to 2 /2 hours is described in this invention, it is set forth that the potential used may be of the order of about 1000 volts to about 25,000 volts at a frequency from zero cycles per second to about 100,000 cycles per second. Further the length of time for treatment of the materials may be of the order of about 1 hour to about 24 hours.

Obviously many modifications and variations of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof, and only such limitations should be imposed as are indicated in the appended claims.

We claim:

1. A method of treating a gamma ray detector of the electrical pulse-producing type in which anode and cathode electrodes are disposed in a housing adapted to contain an ionizable gaseous filling, which comprises subjecting said electrodes to a vacuum while simultaneously impressing between said electrodes an alternating current potential, to establish rapidly reversing large magnitude electrical fields within the detector between said anode and said cathode removing said potential and filling said housing with an ionizable gas.

2. A method of treating a gamma ray detector of the electrical pulse-producing type in which anode and cathode electrodes are disposed in a housing adapted to contain an ionizable gaseous filling, which comprises subjecting said electrodes to a vacuum of not more than one micron of mercury pressure while simultaneously impressing between said electrodes an alternating current potential, to establish rapidly reversing large magnitude electrical fields within the detector between said anode and said cathode removing said potential and filling said housing with an ionizable gas.

3. A method of treating a gamma ray detector of the electrical pulse-producing type in which anode and cathode electrodes are disposed in a housing adapted to contain an ionizable gaseous filling, which comprises subjecting said electrodes to a vacuum while simultaneously impressing between said electrodes an alternating current potential of the order of 6000 to 8000 volts, to establish rapidly reversing large magnitude electrical fields within the detector between said anode and said cathode removing said potential and filling said housing with an ionizable gas.

4. A method of treating a gamma ray detector of the electrical pulse-producing type in which anode and cathode electrodes are disposed in a housing adapted to contain a gaseous filling, which comprises subjecting said electrodes to a vacuum of less than one micron of mercury pressure while simultaneously impressing between said electrodes an alternating current potential of the order of 6000 to 8000 volts and having a frequency of about 60 cycles, continuing this treatment for approximately two hours, to establish rapidly reversing large magnitude electrical fields within the detector between said anode and said cathode removing said potential and filling said housing with an ionizable gas.

5. A method of treating a gamma ray detector of the electrical pulse-producing type having anode and cathode electrodes which comprises subjecting said electrodes to a vacuum of not more than one micron of mercury pressure while simultaneously impressing between said electrodes an alternating current potential of the order of 6000 to 8000 volts, to establish rapidly reversing large magnitude electrical fields Within the detector between said anode and said cathode removing said potential and placing the electrodes in an ionizable gaseous atmosphere.

6. A method of treating a gamma ray detector of the electrical pulse-producing type having anode and cathode electrodes which comprises subjecting said electrodes to a vacuum having an absolute pressure of not more than one micron of mercury while simultaneously impressing between said electrodes an alternating current potential of the order of 6000 to 8000 volts and a frequency of about cycles, continuing this treatment for from 1 /2 to 2 /2 hours, to establish rapidly reversing large magnitude electrical fields within the detector between said anode and said cathode removing said potential and placing the electrodes in an ionizable gaseous atmosphere.

References Cited in the file of this patent UNITED STATES PATENTS 1,374,679 Pratt Apr. 12, 1921 1,565,857 Kelly Dec. 15, 1925 1,929,212 ONeill Oct. 3, 1933 1,974,956 Haficke Sept. 25, 1934 2,403,745 Norton July 9, 1946 2,574,632 Engelkemeir et al. Nov. 13, 1951 

